Recently, compressed fuels have received increased interest for development as an alternative fuel source for gasoline. One such compressed fuel is compressed natural gas which provides a stable fuel source and provides a cost-effective contribution to cleaner mobility.
Previous automotive uses for compressed natural gas, as an alternative to gasoline, in internal combustion engine have been primarily limited to indirect (port) injection fuel systems in which a port fuel injector injects the compressed natural gas into an air flow intake. In indirect injection, the injected fuel and air from the air intake is drawn into the combustion chamber due to a vacuum caused by the downward stroke of the piston when the intake valve is opened.
However, the advantages in increased fuel efficiency and higher power output of direct injection systems over port injection fuel systems have led to an increased focus on direct injection systems. As such, there has been an increased demand for the use of alternative fuels in direct injection fuel systems to combine the benefits of alternative compressed gas fuels, such as compressed natural gas, with the increased fuel efficiency and higher power output of direct injection systems. In previously known gasoline direct injection engine systems, gasoline is injected at high pressure in the range of 1,600 psi. In direct injection gasoline engines the injection timing is limited during the compression stroke of the piston.
There are disadvantages of direct fuel injection systems. One particular disadvantage of the direct injection fuel systems is the added expense due to the increase in the required resiliency of the injectors. Direct injection fuel injectors are disposed partially within the cylinder which exposes the injectors to the intense heat and pressure of combustion. As such, there is an increase in the cost of direct fuel injectors as compared with indirect fuel injectors.
A limiting factor in utilizing compressed natural gas in a direct injection fuel system is the pressure regulation of the compressed natural gas. In the fuel storage tank, the compressed natural gas is stored at a high pressure to maximize the stored volume of the compressed natural gas and the packaging requirements of the vehicle. The stored high pressure compressed natural gas is first depressurized by a regulator to <300 psi. In order to utilize direct injection, the decreased pressure of the natural gas will once again undergo an increase in pressure. This recompression of the fuel is costly in terms of recompression inefficiency due to the packaging and additional components required to recompress the fuel which can lead to a reduction in engine efficiency. Further, as low pressure compressed natural gas fuel injectors are unable to withstand blowback pressure during the combustion stroke of the engine, when located in the combustion chamber, costly direct injection compressed natural gas injectors would be required to withstand the heat and pressure of combustion.
Thus, there exists a need for a cost effective and simple conversion for utilizing low pressure compressed gas fuels in direct injection.